1. Field of the Invention
This invention relates to an optical apparatus for reshaping a generally rectangular cross-section beam of light to a generally square beam. This apparatus has particular application to lasers which have thin, flat ain mediums.
2. Description of the Prior Art
A new type of CO.sub.2 laser (see U.S. Pat. 4,755,999 by the author herein) has a thin, flat amplification volume. This usually produces a rectangular output beam which exhibits two different divergence characteristics parallel and perpendicular to the wide dimension of the rectangular beam. For example, the laser beam typically exhibits greater divergence in the plane parallel to the narrow dimension of the beam compared to the plane parallel to the wide dimension of the beam. This beam exhibits a high degree of spatial coherence, but needs to be reshaped for most applications.
In the past, rectangular output beams have been corrected and reshaped using either cylindrical optics or prisms. Those skilled in the art know that cylindrical lenses or mirrors can introduce a convergence or divergence in one dimension without affecting the orthogonal dimension. Therefore, a rectangular beam with two different divergences can be corrected using cylindrical optics. It is also known that a prism can have the effect of changing the dimension and divergence of an optical beam in one plane, while leaving the other orthogonal plane unaffected. For example, a prism made out of germanium, with an index of refraction of 4 and with a 14 degree wedge angle, can be made to introduce a factor of 4 expansion or contraction in one plane, while leaving the orthogonal plane of the beam unaffected. This change in dimension of the laser beam is also accompanied by an increase or decrease in the divergence of the beam depending on whether the beam was expanded or contracted respectively. Prism correction often is not practical because material limitations and polarization problems limit the usefulness.
Both the cylindrical optics and the high index refraction prisms can be considerably more expensive than optics with spherical curvatures. Cylindrical optics often have to be combined in a multi-element optical system to produce the desired effect. This invention describes the use of spherical reflectors, used off axis, to reshape and collimate these rectangular laser beams.